This invention is related to a rod seal system and particularly to an improved rod seal system for heat engine applications, such as Stirling cycle engine applications. The improved rod seal system isolates a high pressure area from a low pressure area through the use of a pair of rod seals that define a buffer area. The rod seal system utilizes a working gas conduit connecting the buffer area and the high pressure area and a pump that is charged with working gas at relatively low pressure and releases working gas at relatively high pressure when actuated, thereby forcing working gas received from the buffer area back into the high pressure area through the working gas conduit.
Rod seals are used in various machine applications and are typically used to prevent fluids from moving from one area of a machine to another area of the machine when a rod extends between these areas. A number of rod seal designs are known for use under a variety of different types of operating conditions.
While simple and effective rod seal designs are known for use in rigid or static applications, designing a rod seal that provides adequate sealing becomes significantly more complicated if an effective seal must be maintained as the rod moves with respect to the rest of the machine, such as by rotating (the motion of a typical drive shaft) or by reciprocating (the motion of a typical piston rod). Further design complications are introduced if the rod seal is expected to provide a long service free life and the seal must prevent the migration of fluids under a variety of operating conditions, such as during machine warm up and cool down. Additional design complications arise if the areas separated by the rod seal have a significant difference in pressure. This pressure differential creates significant stresses on the rod seal that must be compensated for to provide proper sealing. This pressure differential is particularly difficult to compensate for if the rod seal is expected to prevent losses of a highly permeable gas such as hydrogen. At the high working gas temperatures and pressures used within heat engines, hydrogen gas will permeate through a wide variety of engineering materials, including metal alloys.
The inventive rod seal system has been particularly designed to operate in an environment in which a high pressure area and a low pressure area in a heat engine are spanned by a reciprocating rod, such as by a piston rod. The high pressure area is typically a section within the cylinder walls of the heat engine. Working gas, such as hydrogen, undergoes a cyclical variation in pressure within the cylinder walls which produces a reciprocating motion in a piston positioned within the cylinder walls.
The low pressure area is typically an area within the drivecase of the heat engine. In this area, the reciprocating motion of the piston rod can be converted to rotational motion of a drive shaft, such as by a swashplate mechanism assembly. To reduce the number of areas of the heat engine that may be considered a "pressure vessel" by certifying organizations and others evaluating the heat engine from a safety perspective, the high pressure working fluid in the heat engine is preferably confined to the maximum extent possible to the opposing ends of the cylinder walls and the associated heat transfer devices and passageways. The pressure within the drivecase and the remaining areas of the heat engine can then be maintained at a low pressure, preferably at approximately atmospheric pressure. While isolating these areas substantially reduces the required burst strength of the drivecase and other associated components, it substantially increases the pressure differential between the working gas within the cylinder walls and the gas (typically air) within the drivecase. This increased pressure differential substantially increases the difficulty of effectively sealing the high pressure area within the cylinder walls from the low pressure area within the drivecase.
The inventive rod seal system effectively isolates the high pressure working gas filled area from the low pressure area by utilizing a pair of rod seals which form a buffer area between the high pressure area and the low pressure area. A pump positioned with a working gas conduit is used to force working gas that leaks past one rod seal to the buffer area back into the high pressure area. The pump removes working gas from the buffer area by absorbing the working gas at relatively low pressure. When the pump is actuated, it releases the working gas and reintroduces the working gas into the high pressure area. The pump is connected to a controller which actuates the pump when a sensor in fluid communication with the buffer area determines that the pressure in the buffer area has exceeded a certain level. Check valves are located in the working gas conduit on opposing sides of the pump to assure that the flow of working gas through the working gas conduit only occurs in a single direction, i.e. from the buffer area to the high pressure area. A working gas storage container can also be installed within the working gas conduit to increase the quantity of working gas that can be stored within the working gas conduit and increase the effective size of the buffer area.
The low pressure area of the heat engine will typically contain a lubricant, such as oil, as well as air. The outer peripheral surface of the piston rod may, for instance, be coated with a thin film of the lubricant and this lubricant may tend to accumulate within the buffer area as the piston rod reciprocates with respect to the rod seals. In these cases, some type of lubricant/working gas separator, such as an oil separating membrane, can be positioned within the working gas conduit and used to prevent the lubricant from traveling through the working gas conduit toward the high pressure area. A lubricant conduit can be used to return the lubricant isolated by the lubricant/working gas separator to the low pressure area. The lubricant conduit can include a lubricant trap which stores lubricant before it is returned to the low pressure area and a lubricant return valve which allows the lubricant to enter the low pressure area from the lubricant conduit.
The heat engine environment of the current rod seal system can comprise a Stirling cycle engine similar to those previously developed by the assignee of the present invention, Stirling Thermal Motors, Inc., including those described in U.S. Pat. Nos. 4,481,771; 4,532,855; 4,615,261; 4,579,046; 4,669,736; 4,836,094; 4,885,980; 4,707,990; 4,439,169; 4,994,004; 4,977,742; 4,074,114; 4,966,841; and 5,611,021; which are hereby incorporated by reference. Basic features of many of the Stirling cycle engines described in the above referenced patents may be implemented in connection with a heat engine incorporating the present invention.
The inventive rod seal system allows for a more effective seal to be formed between the high pressure area and the low pressure area than is possible with a conventional system having a single sliding contact rod seal and results in reduced frictional power losses and extended rod seal service lives compared to conventional systems.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.